1. Field of the Invention
The present invention relates to a reflective liquid crystal display device which is widely used as display devices for computers, portable information terminals, electronic calculators, electronic organizers, and the like.
2. Description of the Related Art
Reflective liquid crystal display devices have been widely used for various types of portable apparatuses due to their low power consumption. In recent years, with the sophistication of information, a demand for performing color display for such portable apparatuses has increased. As such, reflective color liquid crystal display devices have been actively developed.
FIG. 14 is a schematic plan view of a configuration of a conventional active matrix type reflective liquid crystal display device. FIG. 15 is a plan view illustrating the conventional display device of FIG. 14 in more detail. FIG. 16 is a sectional view illustrating the structure of an amorphous silicon (a-Si) thin film transistor (TFT) which is used as an active element for the reflective liquid crystal display device.
The configuration and the fabrication method of the conventional reflective liquid crystal display device will be described with respect to FIGS. 14 to 16.
First, a metal film is formed on a glass substrate by sputtering and patterned by photolithography and etching, to form gate bus lines (scanning lines) 20 and gate electrodes 21 of TFTs 26.
Then, a gate insulating film 40, a semiconductor layer 41, and a contact layer 42 are sequentially formed and patterned so that semiconductor layers 41 and contact layers 42 are at least partially formed at portions above the gate electrodes 21.
Thereafter, a metal for source bus lines (signal lines) 30 is deposited by sputtering and patterned, to form the source bus lines 30 as well as source electrodes 31 and drain electrodes 32 of the TFTs 26. Subsequently, the portions of the contact layers 42 located above channel portions of the TFTs 26 are removed.
An interlayer insulating film 50 is formed over the resultant substrate to flatten the uneven top surface of the substrate. Contact holes 33 are then formed through the depth of the interlayer insulating film 50 at positions above the drain electrodes 32.
Finally, a metal thin film is formed and patterned to form reflective pixel electrodes 60. The pixel electrodes 60 are in electrical contact with the corresponding drain electrodes 32 via the contact holes 33.
Thus, an active matrix substrate is fabricated. The resultant active matrix substrate is bonded together with a counter substrate including a counter electrode formed on substantially the entire surface thereof with a predetermined -space therebetween. A liquid crystal material is injected into the space between the substrates and forms a seal therebetween, thereby to complete the reflective liquid crystal display device.
As shown in FIG. 14, the illustrated conventional reflective liquid crystal display device employs a pixel arrangement called a delta arrangement, which is advantageous, in general, in the display of video images, static images, and the like. When the pixel electrodes 60 are formed to overlap the adjacent gate bus lines 20 having the interlayer insulating film therebetween, a storage capacitance (Cs) is produced at each of the overlap portions and the area of each pixel electrode 60 increases. This overlap structure therefore serves to increase the amount of reflected light from the display device.
However, the above configuration has the following problem. Since each of the above Cs portions is recognized as part of a pixel region, the resultant pixel region has a shape as shown in FIG. 18, which is composed of a pixel portion 60a of substantially a rectangular shape (the shape of a pixel electrode obtained when no Cs portion is formed as shown in FIG. 17) and an additional pixel portion (extending portion) 60b corresponding to the Cs portion extended from the pixel portion 60a to a considerable extent. With this shape of the pixel electrodes, when the display screen is divided into sections Q, P, and O defined by vertically dashed lines as shown in FIG. 18, and the occupation of the area of red (R) pixels, for example, in the entire area of each divided section (hereinafter, referred to as the area occupation of R pixels, for example) is compared with those of other divided sections, the result of Q&gt;P&gt;O is obtained as will be described hereinbelow. FIG. 18 illustrates only three rows of pixels as an example, and thus the center section Q among the three is shown as including only one red (R) pixel. It should be noted that since the same pattern of pixel arrangement continues in the vertical direction, if four rows of pixels were taken into consideration, the area occupation of R pixels would have been the same for the three sections Q. The above description regarding the area occupation of R pixels is also applicable to other colors G and B. As shown in FIGS. 14 and 15, the TFTs 26 are formed on the right and left sides of each source bus line 30 alternately, and the pixels of the same color are connected to each source bus line 30.
FIG. 19A is a simplified illustration of the aforementioned area occupation of R pixels. Referring to FIG. 19A, while section Q has a high area occupation of red pixels, section P has a reduced area occupation since only part of the additional pixel portions 60b where the red pixels overlap the gate bus lines 20 are included therein, and section O includes no red pixel portions therein.
Thus, as will be observed from FIG. 19A, in the conventional reflective liquid crystal display device, a pattern of the sections Q, P, Q, and O constitutes one pattern cycle which corresponds to three pixel regions. This means that one pitch (one pattern cycle) of color shade is three times as large as the pixel pitch. Accordingly, when the pitch of one pixel is several tens of micrometers or more, the difference in the density (i.e., occupation area) of each color is visually recognized as vertical stripes, and a vertical stripe pattern is observed at a pitch three times as large as the pixel pitch, i.e., at a pitch of approximately 0.5 mm. This degrades the display quality. More specifically, when the pixel arrangement shown in FIGS. 14 and 15 is employed, such a vertical stripe pattern is observed on a screen for image display which has a size of 3 inches diagonally and includes tens of thousands of pixels.